Method of making grid structures



JUlV 25, G. F. MURPHY METHOD oF MAKING GRID sTaucTUREs Fied July '1, 1930 o o o o o o o o 0 o Q o o o o o o o o fo o o o o o o o o o o o o o o o o o o gggg Inventor:

George F. Murphy,

by Hts Attorney.

Patented July 25, 1933 q UNITED STATES PAT-ENT OFFICE GEORGE F. MURPHY, OF SCEENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OIIE NEW YORK l METHOD 0F MAKING GRID STRUCTURES Application led July 1, 1930. SerialV No.4 485,163.

The present invention relates to electrical discharge apparatus, more particularly to thermionic devices containing an ionizable medium and provided with a source of electrons, a cooperating anode and an electrostatic control member (or grid). The present invention is directed moreespecially to the con uration and manufacture of a control mem er.

In devices of this character, the pressure of the ionizable medium, which ordinarily comprises mercury vapor or a fixed gas, and the impressed voltages are such that a discharge of arc-like character is produced between Athe electrodes, initiation of the discharge being controlled by the bias on the grid. After the -arc is started, the id loses control and can neither modulate, imit nor extinguish the arc.` The starting of the arc may be repeated indefinitely because while the discharge cannot be extinguished by the grid, it can be stopped by removing the anode voltage. Upon reapp ing this Voltage, the grid again determines whether the arc will start, also the arc-starting po-L sition in the cycle, and bya continued repetition of this process the grid can control the average plate current over a period of time. A practical method of obtaining interruption-ofthe .plate circuit is to emplby alternating current, although it will. be understood that direct current may also be `utilized in case proper circuit interrupting means are employed. Devices of this character have been described in an article en titled Hot cathode thyratrons in the General Electric Review, vol. 32, No. 2, April, 1 929, pp. 213-223 inclusive and an article entitled Hot cathode thyratrons in the Greneral` Electric Review, July, 1929, vol. 32, No. 7, pp. 393-4. Such devices find utility in circuits where large currents arev to be cont-rolled by the applicationof a very small amount of energy to the grid.

In tubes Q'f this character, the id or electrostatic kcontrol member surroun s the anode and is provided'with perforations, the size and spacing of which determine some of the 'operating `characteristics of the tube such as the grid control ratio, etc. In order to 'obtain tubes whichfperform not only in a with'extremely small openings which must v5&5

be of uniform size and shape. Nichrome, i. e. nickel-chromium alloy, is usually employed for grid material on account of its reluctance to emit secondary electrons and for other reasons, but the metal is tough to work. It has. been found practically impossible to punch a sheet of nichrome with oles much less than .025 inches diameter which are altogether too large for tubesof certain characteristics; the small punches in-. variably break under these conditions before penetrating the entire thickness of the sheet. Drilling holes of this size or /less would not of course be feasible from a quantity production standpoint:l

An object of the resent invention is to provide an improve grid and method of making the same, the grid being character-r ized by openin s of minute and accurate dimension andt e method of fabrication be- 16 ing adapted to metal whichv-is unusually tou h. This object is attained, Ain brief, by per oratin sheet `metal of greater than the required t ickness vwith o nings of larger than the required size, t en reducing the thickness of the 'metal by rolling 'v or otherwise to the requisite thickness whereupon the metal in cold owing will cause a reduction in the size of each o ning to the requireddimension, also a c 'an in the center-to-cente spacing. Other o jects and features will be apparent as the specification is perused in connection with the accompanymg drawing. As an example of an additional feature it has been found that the rolling .process oliers a convenient method of obtaining openings of elo ated shape, such as diamond, ellipse, etc., w en the rolling is performed in a direction in which the elongation is desiredng y vie'wof 'a tube "ethcf arc discharge type,

partly broken away, and employing an electrostatic control member im roved -m accordance with my Invention; 1g. 2 is a view 10 partly in elevation and partly in section of a perforating die; Fig. 3 -is a plan view of the die; Fig. 4 is a section of perforated grid material before rolling, while Fig. 5 illustrates the material after rolling in a longitudinal direction and de icting an elolgation of each openin `in that direction; ig 6 shows .the effect o rolling the material in transverse and longitudinal directions which produces a marked reduction in the size of the opening; Fig. 7 is a modified. shape of perforation in the sheet metal before rolling,

and Fig. 8 shows the elongating effect of rolling the square-perforated sheet in one direction to produce diamond-shaped openings. Fig. 9 illustrates the rolling operation.

In Fig. 1, numeral 1 designates an evacuated glass envelope provided at one end with a base 2 which has a plurality of contact pins 3.. A reentrant stem et extends within the envelope from the base end and serves to support a cathode 5 and Agrid member 6. The cathode may be of* the indirectly heated type with a filament 7 positioned in the cylinder 5; the latter may be coated on the exterior with an electron emitting material such as barium carbonate. One end of the filament is shown as connected to the closed top of cylinder"5 so that one of the rods-8 which support the latter from the press may also serve as a leading-in conductor for the filament. The other end of the filament is independently supported by a wire 9. v The support rods 8 pass through andare connected to a metal plate 10 which protects the stem from heat radiation and from positive ion bombardment. The wire 9 is extended through a large opening (not shown) in the' plate 10, thereby being insulated from the latter.

The grid or electrostatic control member, to which the present invention is directed, is constituted of mesh or perforated material and may take the form of a hollow cylinder positioned within a drawn-down portion ofthe envelope. The grid is supported from the stem 9L by means of equi-distantly spaced rods l1, one end of which is bent and welded to a circular perforated'plate 12 fitted with.v

in the grid. The other end of the rods is secured to a screw clamp 13 from which a lead is taken to one of the contact pins 3.

There is a plate-like anode 14, carbonized to reduce secondary emission and to increase heat radiation, mounted within the grid by means of conductor 15 which is connected to an exterior contact 16v at the end of the envelope remote from the base. The tube is fabricated and assembled in the usual manner, the customary bakeout and heat treatments given, also a high degree of evacua-4 tion, after which a source of positive ions such as inert gas or a drop of mercury (not shown) is admitted to the envelope. A device of this character when energized by alternating Vcurrent of suitable voltage operates as an electrostatically controlled arc discharge rectifier in which a small charge applied to the grid may control an average plate current of many amperes.

As stated hereinbefore, the grid control ratio, i. e. the ratio of the grid voltage nec essary to restrain initiation of the arc discharge with respect to the plate voltage during the positive portion of the alternating current cycle is a function of the size and shape of the apertures in the grid, also the distance between apertures. It frequently happens that a control ratio is desired which necessitates a very small opening, less than .025 inches diameter and of accurate shape and size. When the grid is made of tough metal such as nichrome, it has been found, prior to my invention, that the procurement of holes less than .025 inches diameter insheet metal .010 inches thick is practically impossible.

However, in accordance with my invention, I have discovered that if holes of a size larger than desired and suiiciently large to be readily punched are cut in sheet metal having a thickness correspondingly greater, the metal may be rolled to the proper thickness, and at the same time, the holes reduced to the proper size. It will be understood that the filial thickness of the grid material is ngt so critical in determining the control ratio as the size of opening so that when the proper opening has been obtained by, rolling, the Vlatter operation may be stopped although, if desired, the thickness of metal before rolling may be predetermined by experiment or otherwise to produce the desired final thickness when the correct size of hole has-been reached by rolling.

Fig. 2 shows a typical press for punching the holes in sheet metal before rolling. The press is constituted of two parts 17 and 18, the upper of which is given a reciprocating vertical motion lby any suitable and well known mechanism and carries two or more rows of punches 19 made preferably of high speed steel such as tungsten drill rod. The movable part 17 comprises two plate members 20, 21 secured together by screws 22 so that when taken apart the punches may be inserted into countersunk holes provided in member 21 and securely held in proper ositi'on by the abutting upper plate mem er 20. The lower or stationary part 18 lof the press lmay comprise two plate members 28, 24, which lare held apart by spacers 25 to form a slot 26 through which the sheet metal is fed in any suitable manner. The part .18 contains a plurality of openings in alignment with the punches in the upperA part and has its various sections secured together by screws 27. A

Fig. 3 depicts the manner in which the sheet metal may be punched, two rows be- 1,919,9M A v 3 ing cut or sheared at one operation after which the sheet is moved downwardly, as shown, through the slot 26 a predetermined distance in order to plunch the next pair of 5 rows of holes. The s eet metal is of greater than the required thickness so that the holes' may .be quite large and the punches correspendingly more rugged and having less tendency to break during the shearing operation. It has been found that punches of .0259 inches diameter and larger will readily vand consistenti penetrate sheet nichrome and other equa ytough stock of thicknesses as great as .010 inches after which the metal 15 may be rolled down to..005 inches thick or other desired dimension.

Fig. 4 represents a section of sheet metal so punched. When the sheet has been rolled in one direction in the manner shown in Fig. 9, the openings which, originally were circular now become elliptical or elongated as indicated in Fig. 5, since the metal cold flows due to the pressure exercised by the rolls 27 and uniformly distorts the shape 25 of each opening. The final step is to feed the sheet metal through. the rolls 27 in a direction transverse to the first direction, which operation tends to restore the original shape of each opening by causing further 3 low of the metal, andwhat is more important, to reduce -the size of each opening. These operations are carried out in such a manner that after the final roll the openings in the sheet metal are of the requisite 5 size, also the shape of each hole conforms as closely as possible to that originally punch- 'ed and the thickness of metal is substantially that required, although as pointed out hereinbefore, the matter of proper thickness o is the least important of the various requisites. These factors may be readily determined b experiment or calculation and are accurate y reproducible in the same sheet metal. y LIt will be understood that if desired, the

rolling operation may be stopped after passing the metal sheets thro lrthe rolls in one direction so as to obtain e ongated opening; since apertures of any given shape may 5 desirable to endow the tube with certain o erating characteristics. The shape of t e openings ma be, minutely varied if desired,

by rolling t e sheet more in one direction than in another direction until the proper size and shape of opening is obtained.

The rolling process is also useful in obo taining shapes of apertures which would be rohibitive from a cost standpoint regardess of size of opening and regardless of the ease with which the metal may be worked. Thus, in Fig. 7 s uare apertures 75 have been punched with rea ily procurable square punches and upon rolling in one direction diamond-shaped openings appear, as indicated in Fig. 8- which could not have been punched economically due to the ex- 8 pense of fashioning diamondshaped punches;

It will be evident from the foregoing that in additionyto `enabling the procurement of minute holes in relatively tough metal which` is unattainable by prior art methods, my invention allows an enhancement of the accuracy of the shape, sizeand spacing of the holes because the latter are more easilfy and accuratel punched when they are o substantial imension rior to the rolling operation. Due to this increased degree of accuracy the holes as finally obtained are accuratel re roducible.

at claim as new and desire to secure by Letters Patent of the United States, is,-

1. The method of fabricating grid material which consists in perforating sheet Y metal with o enings greater than the required size, t en rolling the metal in two irections angular to one another in order to reduce the size of the openings and to e obtain a predetermined shape of opening.

2. In the art of fabricating sheet metal 'd material which is provided with open 105 lngs of a sha different from the shape of standard per orating punches, the methodwhich consists in perforating sheet metal of greater thickness than the final required thickness with openings readily rocurable by punches of standard sha an then rolling the metal sheet a pre etermlned number of times in a plurality of directions -until the linal proper thickness of metal and the ro r shape of opening are obtained.

. P GEORGE F. MURPHY. 

